The present invention relates to disconnectable unions for fluid piping and more particularly to an arrangement for establishing a superior leak resistant joint between piping components.
In 1976, the piping code document ANSI B31.3-1976 entitled "Chemical Plant and Petroleum Refinery Piping" was revised to reflect growing concern with the conveyance of hazardous and toxic fluids. It includes a service category designated "Category M" which is defined as "a toxic fluid service in which exposure to very small quantities in the environment can produce serious irreversible harm to persons on breathing or bodily contact, even when prompt restorative measures are taken." The specifications promulgated for meeting such service requirements are among the most stringent ever devised and operate on the theory that acceptable piping should be essentially leakproof.
In the absolute sense, nothing is leakproof since everything is affected by the passage of time. Therefore, one approach has been to provide concentric containment structures with provision to detect failure of the inner structure before the outer structure is induced to fail. For example, it is asserted in an article entitled "Toxicology and Process Design" by Walter R. Payne, Chemical Engineering Apr. 24, 1978, pp. 83 to 85, that "piping should be joined by welding if possible; in other cases, the flanges or couplings should carry shields or guards to prevent the possibility of a splash if the joint fails. For every toxic or otherwise hazardous material, the designer should consider a concentric-pipe system, so that failure of the chemical carrying inner pipe can be quickly recognized." An illustration of a concentric-pipe joint appears in said article wherein a bolted flange joint interconnects the ends of two pipe components. The seal between the flanges appears to be a conventional gasket while the entire joint is shrouded in a protective shield, the assumption being that the joint may fail. In fact, flange joints have always been the weak link in any piping system.
In Swiss Pat. No. 373,934 issued Jan. 31, 1964 there is disclosed a sealing arrangement for a flanged pipe joint wherein the seal consists of a hard metal ring coated with a layer of softer metal such as silver and provided with a plurality of concentric circular grooves formed in its face, providing the structure with plural sealing zones, whereby leakage past one zone into a groove can be detected. In one embodiment protection against leaks in the seal is further increased by welding together and to the flange faces a pair of short tubular elements referred to as tabs or tongues which in essence encloses the seal within a rigid pipe and converts the joint from one that is disconnectable or temporary to one that is permanent.
Direct welding of pipe, while avoiding certain weaknesses of a flange joint, has its own limitations not the least of which is cost of producing a welded joint and its incompatibility with the concept of disconnectability; hence, often a preference for flange joints. However, flange joints, as presently known, are subject to failure for various reasons, most of which involve loss of sealing integrity. The sealing integrity of a standard flange joint is affected adversely by: improper installation, i.e., use of wrong bolts, wrong torque, etc.; bolt fracture due to over-torque; bolt corrosion, and metal creep in the bolts. Misalignment of piping during installation is a major cause of overstressed bolts. Other adverse factors are: gasket cold flow; gasket deterioration with age; gasket shrinkage due to low temperature; gasket deformation under heat cycling; gasket degradation by conveyed fluids; pipe bending loads and tensile loads; vibration; shock loads and water hammer; and proximity to external overheat conditions. Of course, the joints are also subject to failure upon the occurrence of fire, flood, earthquake or other catastrophic conditions.
In the discussion that follows two distinctly different types of fluid seals will be mentioned. Certain seals are established by bringing together confronting surfaces of the pipe joint components or union members either directly or with a gasket type sealing member interposed therebetween. In either case the effectiveness of the seal that is established depends upon the forces with which the parts are brought together and ultimately on the pre-loading developed between the sealing surfaces or within the seal member itself. In a flange joint, for example, with a simple gasket sandwiched between the flanges, the pre-loading of the gasket, and the effectiveness of the fluid seal, will depend initially on the torque developed in the flange bolts. Because of pipe misalignment the individual flange bolts are often subjected to unequal stress. Once installed, however, the pre-loading will experience diminution upon the occurrence of any one or more of the following phenomena: metal creep in the bolts, gasket cold flow, gasket deterioration with age, gasket shrinkage due to low temperaure (assuming the absence of compensating bolt shrinkage), gasket deformation under heat cycling and gasket degradation by conveyed fluid. Pipe bending loads and tensile load can also cause reduction in seal pre-loading but this may not be uniform circumferentially throughout the seal. The foregoing enumeration should not be considered as exhaustive of the possibilities but merely exemplary for the purpose of conveying the meaning to be attached to the expression "pre-loading" as used herein. Another way of looking at it is to consider the seals as being gap sensitive, i.e., sensitive to the gap between the union members that are being sealed.
Contrasted with the above are seals of the type that are not readily categorized but which are in essence not gap sensitive. The nature of these latter seals will appear from the detailed description of the invention.
With the foregoing in mind, it is an object of the present invention to provide a pipe union which has concentric piping integrity, retains the disconnectability of standard union joints, can be assembled quickly and economically, and affords reliable safety without the need for bulky surrounding protective shields as well as continuity of operation which shields cannot provide.